Multilayer pipe and method for manufacturing one

ABSTRACT

A multilayer pipe which comprises at least an inner layer and an outer layer. The inner layer is made of extrudable plastic, such as polyethylene PE, cross-linked polyethylene PEX or polypropylene PP. The outer layer forms a fire-protecting layer and the matrix plastic of the outer layer is also made of extrudable plastic. A foaming agent, which does not react during the manufacture of the pipe, is mixed with the matrix. Further, the outer layer contains at least one additive in such a manner that the outer layer has a pipe-reinforcing component and a fire-retardant component. Most preferably, at least the outer layer is extruded by an extruder having a screw length ratio of less than 10 to its biggest diameter.

The invention relates to a multilayer pipe which comprises at least aninner layer and an outer layer, the inner layer being made of extrudableplastic, the matrix plastic of the outer layer being made of extrudableplastic and the outer layer being arranged to be a fire-protectionlayer.

The invention further relates to a method for manufacturing a multilayerpipe.

Fire-retardant or fire-resistant properties are required in pipes whichare, for instance, used as sprinkler pipes, gas pipes inside buildings,discharge pipes of vehicles and pneumatic and hydraulic pipes inmechanical engineering. U.S. Pat. No. 5,671,780 discloses a multilayerflexible conduit having an outer layer of halogen-free andphosphorus-free thermoplastic material. The intermediate layer containsfire-resistant fibers or textile, and the inner layer is adiffusion-tight layer of a thermoplastic material. U.S. Pat. No.4,942,903 discloses a plastic pipe having an innermost layer ofpolymeric plastic. Heat insulation is arranged outside the innermostlayer and outside the heat insulation, there is a fire-resistant layerwhich at a high temperature forms a ceramic stable fire-protectingphase. U.S. Pat. No. 5,799,705 discloses a structural pipe around whichone or more fire-protecting layers are arranged, the layers comprisingfiber-reinforced resin layers. U.S. Pat. No. 4,788,090 discloses aplastic pipe coated on the outer surface with a thermal insulationcomprising foam. Outside the thermal insulation, there is an outer layercontaining fibers. In a fire, the fibers fuse together increasing themechanical strength of the foam layer. A weakness in the solutionspresented above is, among other (things, that the manufacture of suchpipes is quite difficult. Further, to ensure adequate fire-protectionproperties the pipes must be made quite thick. Adding fire-protectionagents to the pipe materials also decreases the mechanical strength ofthe pipes.

U.S. Pat. No. 5,552,185 discloses plastic articles, such as pipes,equipped with fire enduring properties. The article comprises a corelayer which is made of a polymeric plastic material, and said core layeris coated with a fire-resistant material. The fire-resistant layer ismade up of a thermoplastic material and a char-forming material and anintumescent material. In a fire, the fire-resistant layer forms char andbubbles on top of the core layer. This type of article is quitedifficult to manufacture and the mechanical strength properties of thearticle may require that the wall becomes quite thick.

DE publication 19 504 613 discloses a five-layer pipe having a foamlayer as the second-outermost layer which contains a foaming agent whichdoes not react during the manufacture of the pipe. When the pipe isexposed to heat, the foam layer produces foam. To make the pipemechanically strong enough, it must have several layers, such as anoutermost layer outside the foam layer. This is why the pipe structureis very complex and the pipe difficult and expensive to make. Further,halogen plastics are used in the pipe, which in a fire form dangerouscompounds.

It is an object of the present invention to provide a new type offire-enduring multilayer pipe.

The pipe of the invention is characterized in that the outer layercontains a non-reacted foaming agent and at least one additive in such amanner that the outer layer has a fire-retardant component and apipe-reinforcing component.

Further, the method of the invention is characterized in that the outerlayer of the pipe contains a halogen-free fire-protecting agent and thatat least the outer layer is extruded by an extruder having a screwlength ratio of less than 10 to its biggest diameter.

An essential idea of the invention is that the multilayer pipe comprisesat least an inner layer and an outer layer, the inner layer being madeof extrudable plastic, typically polyolefins, such as polyethylene PE,cross-linked polyethylene PEX or polypropylene PP. The outer layer formsa fire-protecting layer for the inner layer and the matrix plastic ofthe outer layer is also made of extrudable plastic. A foaming agent,such as azodicarbonamide which does not react during the manufacture ofthe pipe, is mixed with the matrix. Further, the outer layer contains atleast one additive in such a manner that the outer layer has apipe-reinforcing component and a fire-retardant component. Thefire-retardant component can be an inorganic filler, such as aluminumtrihydrate ATH, vermiculite, silicate, phosphate or carbonate, oranother corresponding halogen-free fire-protecting agent. Thefire-retardant additive can, at the same time, serve as a reinforcingcomponent, in which case said additive can be a fire-resistant inorganicmaterial, such as short-cut fiberglass or ceramic whiskers fiber. Thereinforcing component can also be a separate additive, in which case itcan, for instance, be an inorganic fiber or a mixture of inorganic ororganic fibers or reinforcing needle- or flake-shaped minerals. Whenexposed to fire, the foaming agent, which did not react during themanufacture of the plastic pipe, reacts and produces foam that makes theouter layer thicker, thus making the layer at said foamed location serveas a very good insulation. Due to the reinforcing component, themechanical strength of the foamed layer also remains quite good.

The invention provides the advantage that the outer layer which protectsthe pipe against fire can be quite thin. Because the outer layer has apipe-reinforcing component, the pipe is as a whole quite mechanicallystrong.

The invention is described in greater detail in the accompanyingdrawings, in which

FIG. 1 is a schematic cross-sectional view of a pipe of the invention,and

FIG. 2 is a schematic cross-sectional view of another pipe of theinvention.

FIG. 1 shows a multilayer pipe having an inner layer 1 and an outerlayer 2. The inner layer 1 is made of extrudable plastic. The materialof the inner layer 1 can be a polyolefin, such as polyethylene PE,cross-linked polyethylene PEX or polypropylene PP. The inner layer 1 canalso be made of another material, such as polyamide PA or acrylonitrilebutadiene-styrene ABS. Mixtures of different materials can also be used.

The matrix plastic of the outer layer 2 can be any extrudable plastic orplastic mixture. The basic material of the outer layer 2 can be apolyolefin or ethylene vinyl acetate EVA, for instance.

A foaming agent, which does not react during the manufacture of thepipe, is mixed with the matrix of the outer layer 2, and the outer layer2 thus contains a non-reacted foaming agent. The foaming agent can, forinstance, be an azodicarbonamide or the like. The outer layer 2 alsocontains an additive in such a manner that the outer layer 2 has afire-retardant component and a pipe-reinforcing component. This additivecan, for instance, be short-cut fiberglass or ceramic whiskers fibers,which at the same time serve as a fire-retardant component and apipe-reinforcing component. This type of non-flammable inorganic agentmakes it possible for the outer layer to expand when the foaming agentreacts in heat, but the additive keeps the outer layer 2 structurallystrong, however. The foaming agent thus reacts at a temperature higherthan the extrusion temperature. The additive thus at the same timeserves as a fire-retardant component and increases the oxygen index ofthe outer layer 2.

The outer layer 2 can also contain inorganic fillers. Severalhalogen-free fire-protecting agents, such as aluminum trihydrate ATH,vermiculite, silicate, phosphate or carbonate which increase the oxygenindex of the outer layer, can be used as fillers. The oxygen index ofthe outer layer is preferably increased to over 30. Magnesium hydroxide,halogenated fire-protecting agents often together with antimony oxide orphosphor-containing fire-protecting agents can also be mixed to theouter layer 2. Carbon and/or carbonaceous agents can also be used asadditives, and they can be quite small in size, in the nanometer-range,for instance.

The outer layer can be reinforced with inorganic fibers or a mixture ofinorganic and organic fibers, for instance. Needle- or flake-likeminerals, such as fiberglass, ceramic whiskers fibers or Mica, can alsobe used as the reinforcing additive.

If desired, the outer layer 2 can also contain an UV stabilizer orthermal stabilizer. The pipe can also have adhesive agents in the layersor between the layers to improve the adhesion of the outer and innerlayers to each other. Coloring agents and/or light-reflecting pigments,such as metal particles, can also be added to the outer layer.

The outer layer 2 can be extruded in such a manner that it can easily bepeeled off from the inner layer 1, as disclosed in U.S. Pat. No.5,794,61. It is then easy and simple to attach the pipe byelectro-welding or some other attaching system.

FIG. 2 shows a pipe, in which an intermediate layer 3 is arrangedbetween the inner layer 1 and the outer layer 2. The intermediate layer3 is a metal layer which is made of thin aluminum, for instance, andwhich can serve as a barrier layer. An inner adhesion layer 4 isarranged between the inner layer 1 and the intermediate layer 3.Correspondingly, an outer adhesion layer 5 can be arranged between theouter layer and the intermediate layer 3. The adhesion layers 4 and 5can be made of an adhesion material available by the trade name Surlyn.The thickness of the inner layer is typically approximately 1 to 10 mm,for instance, the thickness of the intermediate layer is approximately0.1 to 3 mm, and the thickness of the outer layer 2 serving as thefire-protecting layer is approximately 0.2 to 5 mm. The adhesion layers4 and 5 are less than 0.1 mm thick. The total diameter of the pipe thenvaries between 10 and 100 mm. The outer adhesion layer 5, for instance,can be left out, if the outer layer 2 is made of a copolymer plasticwhich has modified end groups for improving adhesion to the underlyingmetal intermediate layer 3.

The metal intermediate layer 3 reinforces the mechanical structure ofthe pipe, but at the same time the intermediate layer 3 spreads thethermalloiad to a wider area owing to its good thermal conductivity. Theheat of a local point-heating flame then does not affect the inner layer1 at one point, but the heat is distributed to a wider area owing to thethermal conductivity of the intermediate layer 3.

The total thickness of a pipe of the invention can be the same as thatof a solid-wall non-fire-protected pipe used for a correspondingpurpose, because the possible metal intermediate layer 3 and thereinforcing agent in the outer layer strengthen the pipe. Alternatively,the total thickness of a pipe of the invention can be bigger than thatof the above-mentioned non-fire-protected pipe, in which case thepeelability of the pipe is preferably utilized in such a manner that theouter layer 2, and possibly the metal intermediate layer 3, too, arepeeled away before the pipe is attached to a mechanical crimpconnection, for instance.

A multilayer pipe of the invention can be extruded in one phase usingseveral extruders and a multilayer cross-head die. The multilayer pipecan also be extruded using a cone-shaped multilayer extruder describedin U.S. Pat. No. 5,387,386, for instance. The use of such a conicalextruder, having a rotor, i.e. extruder screw, length ratio of less than10 to its biggest diameter, is very advantageous, because thefire-protecting materials used in the fire-protecting layer are oftenvery sensitive to shear forces and the shear forces exerted to thematerial in a conical extruder are very small. In addition, the dwelltime of the material in a conical extruder is quite short. A verysignificant advantage is also achieved by the fact that the outer-layerfibers can, in a conical extruder, be oriented to an angle differingfrom the axial direction. It is also possible to make the inner layer(s)first and extrude the fire-protecting layer outermost on the pipe usinga conventional coating technology.

If the innermost layer is made of cross-linked polyethylene PEX, it canpreferably be cross-linked separately before coating. The cross-linkingcan also be done when all layers of the pipe are on top of each other,by using infra-red technology or later on in a separate heating unit.Cross-linked polyethylene can be of any type, i.e. PEX-a, PEX-b orPEX-c. Just the cross-linking method varies in that PEX-a iscross-linked by utilizing heat, PEX-b is cross-linked by moisture: andPEX-c by radiation. The inner layer 1 should preferably be made ofcross-linked polyethylene PEX, because this material is highly heatenduring. Further, due to cross-linking said material will not easilycreep even at high temperatures.

The drawings and the related description are only intended to illustratethe idea of the invention. The invention may vary in detail within thescope of the claims. A multilayer pipe of the invention can be used as asprinkler pipe, gas pipe inside buildings, pressure or discharge pipe ofvehicles and pneumatic and hydraulic pipe in mechanical engineering. Thepipe of the invention can also be used as a protective pipe for optic orother cables. If desired, it is possible to make one or more layersoutside the outer layer serving as the fire-protecting layer.

1-8. (canceled)
 9. A method for manufacturing a multilayer pipe havingat least an inner layer and an outer layer, the method comprising:coating the inner layer formed using extrudable plastic with the outerlayer, wherein the outer layer of the pipe contains a halogen-freefire-protecting agent and at least the outer layer is extruded by anextruder having a screw length ratio of less than 10 to its biggestdiameter.
 10. A method as claimed in claim 9, wherein the outer layercontains reinforcing fibers which are oriented in a conical extruder toan angle differing from the axial direction during the manufacture ofthe pipe.
 11. A method as claimed in claim 9, wherein the outer layercontains a foaming agent and the properties and extrusion temperature ofthe foaming agent are selected in such a manner that the foaming agentdoes not react during the manufacture of the pipe.
 12. A method asclaimed in claim 9, wherein the outer layer contains a foaming agent andthe properties and extrusion temperatures of the foaming agent areselected in such a manner that the foaming agent reacts when the pipe isexposed to fire to convert the outer layer to a foamed layer.
 13. Amethod as claimed in claim 12, wherein the foaming agent comprisesazodicarbonamide.
 14. A method as claimed in claim 9, furthercomprising: prior to the coating, covering the inner layer with anintermediate layer having a heat-conductivity that is higher than thatof the inner layer.
 15. A method as claimed in claim 9, wherein theouter layer contains short-cut fibers and/or ceramic whiskers fibers.16. A method as claimed in claim 9, wherein the inner layer is formedusing a cross-linked polyethylene PEX.